Plasma-assisted chemical reactions have been widely used in the semiconductor and flat panel display industries. One example is plasma-enhanced chemical vapor deposition (PECVD), which is a process that is used in the manufacture of thin film transistors (TFT) for active-matrix liquid crystal displays (AMLCDs). In accordance with PECVD, a substrate is placed in a vacuum deposition chamber that is equipped with a pair of parallel plate electrodes. One of the electrodes, e.g. the lower electrode, generally referred to as a susceptor, holds the substrate. The other electrode, i.e., the upper electrode, functions as a gas inlet manifold or shower head. During deposition, a reactant gas flows into the chamber through the upper electrode and a radio frequency (RF) voltage is applied between the electrodes to produce a plasma within the reactant gas. The plasma causes the reactant gas to decompose and deposit a layer of material onto the surface of the substrate.
One material often deposited is silicon nitride (SiN). SiN is a common material for a gate insulation layer and also for passivation layers due to its ability to resist moisture and sodium contamination. In SiN deposition, as described in U.S. Pat. No. 5,399,387, assigned to the assignee of the present invention and herein incorporated by reference, a plasma of silane (SiH.sub.4) and ammonia (NH.sub.3) gases may be used to deposit SiN according to several reaction paths, for example: EQU SiH.sub.4 +NH.sub.3 .fwdarw.SiNH+3H.sub.2 EQU 3SiH.sub.4 +4NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +12H.sub.2
SiN not only deposits on the substrate but also deposits on the walls and the pumping system. A known in-situ cleaning process may remove the SiN film from the walls by supplying a cleaning gas, often nitrogen fluoride (NF.sub.3), and activating the gas inside the chamber using an RF plasma in order to form pumpable volatile products. This reaction may proceed as follows: ##STR1## The product silicon fluoride (SiF.sub.4) may then react with NH.sub.3 and hydrogen fluoride (HF) in the SiN deposition process to form, for example, ammonium hexafluoride ((NH.sub.4).sub.2 SiF.sub.6). Such products and other similar silicon-containing fluoride products are referred to herein as "white powder", and more generally constitute partially reacted SiN films. This undesirable white powder can condense, for example, in the vacuum pump. The white powder can also condense in the vacuum line connecting the process chamber to the pump (the foreline) and in the vacuum line connecting the pump to the exhaust system (the exhaust line). Finally, the white powder can also condense in the burn box (which treats the exhaust) and on the chamber walls. In the case of the pump and exhaust, the condensation can amount to several kilograms of white powder, often causing pump failure. In the case of the foreline and exhaust line, clogging can occur. The white powder is also a source of undesirable particulates in deposition processes.
Prior plasma in-situ cleaning processes are ineffective at removing the white powder or reducing its occurrence in SiN deposition. In such systems for cleaning the chamber and the exposed components within the chamber, precursor gases are supplied to the chamber. Then, by locally applying a glow discharge plasma to the precursor gases within the chamber, reactive species are generated. The reactive species clean the chamber surfaces by forming volatile compounds with the process deposit on those surfaces. This plasma in-situ cleaning generally does not remove the white powder, regular maintenance of the pump and exhaust is still required.
New enhanced cleaning systems are developed for removing the white powder. For example, some enhanced systems introduce an additional plasma source between the process chamber and the pump. In another example, traps are introduced between the pump and the process chamber or after the pump (in the exhaust line). However, these methods are also ineffective at removing the white powder or reducing its occurrence in SiN deposition.
It is an object of the present invention to reduce the amount of white powder formed during SiN deposition processes. It is a related object to reduce the damage to components that may occur as a result of white powder formation.